COMPOUNDS OF THE TYPE M5X2: Pd5As2, Ni5Si2, AND Ni5P2

Abstract

Space group, lattice parameters, and powder data indicate that Pd5As2 Ni5Si2, and Ni5P2 are structurally related. Single crystal studies have established that Pd5As2 exhibits polytypism. Pd5As2 has a = 7.31 ± 0.01 Å but c-values of 13.7, 10.34, 27.48, and 96.2 Å were observed on different crystals corresponding to a "single layer" repeat distance of 3.43 Å along the c-axis. The last three cells have trigonal symmetry, space group [Formula: see text], [Formula: see text], P3m1 [Formula: see text], or P321 [Formula: see text] and the first cell has hexagonal symmetry, space group P6322[Formula: see text]. The Guinier focussing camera pattern of polycrystalline Pd5As2 can be indexed on a hexagonal cell with a = 7.320 ± 0.002, c = 96 Å. Single crystal and focussing camera studies of Ni5Si2 showed it to be of trigonal symmetry with a = 6.68 ± 0.02, c = 12.28 ± 0.03 Å, and space group P321, P3m1, or [Formula: see text]. A Guinier pattern of Ni5P2 can be indexed with hexagonal parameters a = 13.220 ± 0.002 Å, c = 24.632 ± 0.002 Å; a subcell with a = 6.610 and c = 12.316 Å can be used to index the strong lines. Comparison of the powder data for Pd5As2, Ni5Si2, and Ni5P2 with previously published data for Ni5As2 and Mn5Ge2 shows that the compounds are structurally related.